Many modern refineries devote extraordinary amounts of energy and operating expense to convert most of a whole crude oil feed into high octane gasoline. The crude is fractionated into a virgin naphtha fraction which is usually reformed, and gas oil and/or vacuum gas oil fraction which are catalytically cracked in a fluidized catalytic cracking unit (FCC) unit.
A solid cracking catalyst in a finely divided form, with an average particle size of about 60-75 microns, is used. When well mixed with gas, the catalyst acts like a fluid (hence the designation FCC) and may be circulated in a closed flow loop between a cracking zone and a separate regeneration zone.
The Kellogg Ultra Orthoflow converter, Model F, shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the January 8, 1990 Oil & Gas Journal, is an example of a modern, efficient FCC unit. This design (and many other FCC designs) converts a heavy feed into a spectrum of valuable cracked products in a riser reactor in 4-10 seconds of catalyst residence time.
In the cracking zone, hot catalyst contacts the feed to heat the feed, effect the desired cracking reactions and deposit coke on the catalyst. The catalyst is then separated from cracked products which are removed from the cracking reactor for further processing. The coked catalyst is stripped and then regenerated.
A further description of the catalytic cracking process may be found in the monograph, "Fluid Catalytic Cracking with Zeolite Catalysts", Venuto and Habib, Marcel Dekker, N.Y., 1978, incorporated by reference.
The FCC process is an efficient converter of heavy feed to lighter products, and has some favorable peculiarities. The FCC unit rejects the worst components of the feed as coke and regenerates the catalyst by burning this coke to supply the heat needed for the endothermic cracking reaction. On a volume basis it makes more product than feed. This "swell"--the expanded volume of liquid products after cracking a heavy feed--is one reason the process is so profitable.
FCC produces some of the dirtiest and some of the cleanest fuels. The FCC gasoline is a fairly "dirty" fuel. Although of high octane, the FCC naphtha contains significant amounts of benzene and large amounts of olefins. It will contain a significant amount of sulfur, though this can be reduced by hydrotreating the feed. Hydrotreating the FCC naphtha is not so successful, because hydrotreating enough to remove sulfur and olefins also reduces the octane.
FCC light olefins are potentially some of the cleanest fuels in a refinery. Some refiners consider FCC units to be olefin factories. Most FCC operators use the produced olefins in an HF or sulfuric acid alkylation unit or in an olefin oligomerization or polymerization unit. These fuels, especially the alkylates, which are built up from relatively clean starting materials, have little or no benzene or olefins.
One problem with processing FCC olefins is butadiene. These are extremely reactive in themselves, and undesirable, but also lead to formation of acid soluble oils and excessive acid consumption in, e.g., an HF alkylation unit. Light di-olefins are believed to be primarily the result of thermal rather than catalytic cracking of fresh feed, and are usually considered inherent in the FCC process. Usually butadiene production increases as FCC riser top temperature increases.
Refiners have tried to improve yields in catalytic cracking by changing catalyst and changing reaction conditions. Now essentially all refiners use zeolite cracking catalyst. In the 70's, catalyst with perhaps 10 wt % Y zeolite was common, but now many units use makeup catalyst with 30 to 40 wt % Y zeolite.
Partly in response to the availability of more active catalysts, FCC units have also evolved toward ever shorter reaction times. From dense bed cracking in the 40's and 50's, to hybrid units operating with dense bed and riser cracking, to modern units using all riser cracking, the trend to shorter contact times continues. Many units now practice quick separation of cracked products from spent catalyst exiting the riser to further improve yields. These units (short contact time, quick separation of catalyst and cracked product) usually operate at higher temperatures, which increases unwanted thermal reactions. Increased butadiene content is a measure of unwanted thermal reactions, but is by no means the only undesired side effect of higher temperatures.
The patent literature is replete with references to short contact time cracking, but almost all commercial units operate with riser reactors, with 4 to 10 seconds of catalyst residence time, and with riser top temperatures of about 950.degree. to 1025.degree. F.
Work has been done on ultra-short contact time cracking processes, with less success. Part of the motivation for the ultra-short contact time process was the general belief that higher temperatures and shorter contact time would lead to reduced coke make. Because the activation energy for cracking is higher than for coking, it was thought that higher temperatures would give lower coke selectivity.
The two main variants of short contact time cracking reactors (upflow riser and falling solids) will now be reviewed.